Reporting
There is no exact formula.
There is no exact formula. You need to know what you want to grow and know what pH is required. (The pH is a measure of the acidity of the soil.) Based on that, you might add lime or ground sulfur. You might add peat moss, to help retain moisture. You might add sand (not beach), to help water soak and drain through (not stagnate in) the soil. You might add compost, to help nourish and improve the structure and condition of the soil. The horse (and any other barnyard animal) manure should be aged at least a year. The first question is, what are you going to plant?